Gaseous fuel engines are available from a variety of manufacturers and typically operate with such fuels as methane, ethane, propane, butane and/or mixtures thereof, including natural gas. In recent years increasingly stringent government regulation has forced the engine manufacturers of both gaseous and liquid fuels to changes in operation and/or construction that are increasingly effective toward more complete combustion for effecting both reductions in NO.sub.x emission levels and increased economy of operation. Various approaches toward that end have been disclosed in a variety of U.S. patents, as exemplified by U.S. Pat. Nos. 3,924,582; 3,963,003; and 3,967,611.